1. Field of the Invention
The present invention relates to a method and apparatus for controlling process variables. More particularly the present invention is directed to a method and apparatus applicable to a batch process control whereby the value of the process variable may be quickly brought to its target or setpoint value from the beginning of the process without overshoot.
2. Description of the Prior Art
In the prior art methods, it has been usual that the process control is begun manually and, when the value of process variable has become a predetermined one, in other words, when the deviation of the value of process variable from the setpoint value has decreased into a predetermined range, the manual control is switched to an automatic one to prevent the controlled value from exceeding its limit. However, it is very difficult to preclude such limit excess of the controlled value and to also make the adjusting period a minimum time. The reason for this difficulty is as follows: When the manual output in the manual control is shifted in the direction (maximum or minimum) of increasingly affecting the process variable for the purpose of minimizing the adjusting period, the process variable may quickly reach the target value. On the other hand, the change in the process variable may not follow the abrupt change in the manipulated variable owing to the process characteristic, and the process variable cannot be free from the influence of such abrupt change. As a result, there are involved contradictory characteristics such that the controlled value change inevitably either entails an overshoot when the deviation for determining the timing of switching the control from manual to automatic is minimized or an increase of the adjusting period when the deviation is set larger to avoid overshoot. For this reason, it has been difficult to obtain a good initial characteristic in process control. Particularly, in case of the subject to be controlled whose process characteristic is known with minimum information, it has been difficult to secure a good initial characteristic of the process control.